The sensitivity of a silver halide photographic material is determined by the light absorption factor of a grain, latent image forming efficiency including spectral sensitization efficiency and a minimum size of a latent image.
Of these factors, as to techniques of improving the light absorption factor of a grain, some which are known heretofore are shown below.
Techniques of high aspect ratio tabular grain emulsions disclosed in U.S. Pat. No. 5,494,789, etc., are techniques capable of increasing a dye adsorption amount per one grain because a tabular grain has a larger grain surface area, as a result, the light absorption factor can be improved. However, there are limitations in the increase of the surface area of a grain by heightening an aspect ratio and the like, therefore, a larger sized grain is necessary to improve the light absorption factor of one grain.
In addition to the above, as methods of increasing the grain surface area per one grain, methods of making a pore at a part of a grain are disclosed in JP-A-58-106532 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-60-221320, and a ruffled grain is disclosed in U.S. Pat. No. 4,643,966. However, the forms of grains according to these methods are unstable and accompanied by extreme difficulties in practical use.
Further, U.S. Pat. No. 5,302,499 discloses that a light absorption factor can be improved by constituting the layer structure having spectral sensitization characteristics and optimal grain thicknesses. But the improvement of a light absorption factor by the optimization of the grain thicknesses is at most 10% or so.
Accordingly, for markedly improving a light absorption factor of one grain while maintaining a grain size small with a stable grain form, it is necessary to improve the light absorption factor per unit surface area of a grain. For that sake, it is necessary to heighten the adsorption density of a sensitizing dye, but a generally used spectral sensitizing dye is adsorbed onto a monolayer with almost the closest charging and is adsorbed no more.
Methods which have been proposed for a sensitizing dye to be multilayer adsorbed onto a grain surface are shown below.
In P. B. Gilman, Jr., et al., Photoarachic Science and Engineering, Vol. 20, No. 3, p. 97 (1976), a cationic dye is adsorbed onto the first layer and an anionic dye is adsorbed onto the second layer using electrostatic power.
Further, G. B. Bird, et al., in U.S. Pat. No. 3,622,316, a plurality of dyes are multilayer adsorbed onto silver halide and sensitized by Forster type excitation energy transfer.
However, even these above-described methods could not sufficiently improve the light absorption factor per unit surface area of a silver halide grain, therefore, a further technical development has been required.